1. Field of the Invention
The present invention relates generally to a voice service without discontinuance. In particular, the present invention relates to an apparatus and method for providing a voice service by temporarily camping a User Equipment (UE) of a Third-Generation (3G) network system in a Second-Generation (2G) network system, when the 3G network system temporarily malfunctions and a circuit service, which is the basic voice call transmission/reception function, is unavailable in an area where a user can receive a 3G mobile communication service.
2. Description of the Related Art
At present, the 2G mobile communication service is predominant in the global mobile communication market, and the 3G service is gradually expanding its ground. Accordingly, service providers are expanding a Universal Mobile Telecommunications System (UMTS) Network (UTRAN) and now they are in a transitional period of upgrading the network system from R99/R4 version system, which is a non-High Speed Downlink Packet Access (HSDPA) system, into an R5 system capable of supporting an HSDPA system to competitively provide a more advanced 3.5-Generation (3.5G) HSDPA service. Accordingly, network systems including core commercial networks are being upgraded with respect to software and hardware in various areas. The network systems need to be upgraded continuously to support communication technology that continues to advance.
As a result, the number of errors is gradually increasing in networks interlocking with user equipments. Erroneous operation frequently occurs, when a network vender changes software configurations or when hardware is upgraded. Similar malfunction intermittently occurs even during normal operation due to internal conditions of the network vender.
The malfunction of the networks occurs more frequency in the 3G networks that are commercially set up at present and under progress than in the second-generation networks that are already stabilized. It is desirable with respect to a network and user equipments that a user equipment in support of UMTS receives a service through a 3G network system. This is because, from the point of a network system, a 3G network system has a larger capacity than a 2G network system with respect to the number of subscribers accommodated by one base station and, from the point of users, users can be provided with more additional functions with the 3G service. However, it is troublesome when a user equipment camps in the 3G network system and cannot receive a circuit service, which is the most basic function of the user equipment, when the network system malfunctions and cannot normally provide the service, which is described above. The malfunction intermittently occurs but when it occurs, it causes much inconvenience to the user of the user equipment.
Current 3G user equipments support both 2G service and 3G service but, basically, they register their location in the 3G network system in an area where the 3G service is available, and receive the 3G services.
FIG. 1 illustrates a typical system information broadcasting process. Referring to FIG. 1, a User Equipment (UE) 100 receives a System Information Block (SIB) from a third-generation network system 110, e.g., a UTRAN, and acquires actual information on the network system. The UE 100 decides an available service based on the actual network system information.
The SIB that the user equipment 100 receives from the network system 110 is defined to have 18 types, i.e., from types 1 to 18. Among them, a System Information Block 1 (SIB 1) includes information on a core network for using a circuit service and a packet service.
The UE 100 performs a Location Area Update (LAU) in a Mobile Switching Center/Visitor Location Register (MSC/VLR) of the core network based on circuit switched domain information from the SIB provided from the network system 110, receives circuit service, performs Routing Area Update (RAU) through a Serving General Packet Radio Service (GPRS) Supporting Node (SGSN) within a core network based on Packet Switched Domain, and receives a packet service.
However, when the SIB does not include information needed for the UE 100 to be registered in a circuit switched domain or a packet switched domain due to malfunction of the 3G network system, the UE 100 executes registration in only one of the two domains. In this case, the UE 100 is regarded camping in the 3G network system, although it does not normally receive one service between the circuit service and the packet service.
FIG. 2 illustrates an abnormal system information block of the 3G network system, and FIG. 3 illustrates a normal system information block of the 3G network system.
Referring to FIGS. 2 and 3, the SIB of FIG. 3 includes information on both a packet switched domain and a circuit switched domain, whereas the SIB of FIG. 2 includes only a packet switched domain.
Although the UE 100 receives the abnormal system information block, it can successfully execute the routing area update process and normally receive the packet service because it includes information on the packet switched domain. However, since the UE 100 does not have information on the circuit switched domain, it does not attempt the registration for using the circuit service, i.e., location area update process, and thus it cannot receive the circuit service at all.
In short, the user equipment can use the packet service but it cannot use the circuit service. Moreover, since the user equipment is regarded as normally camping in the 3G network system, it does not display a current state such as ‘service limited’ and ‘service searched’, and a user cannot use the circuit service without knowing the reason.
This phenomenon occurs because the user equipment operates according to standard protocol defining the operation and state. According to the standard protocol (3GPP 25.331), when a user equipment performs a routing area update based on packet switched domain information and does not include information on the circuit switched domain in the system information block, the user equipment informs the state to an upper layer as it is and remains in a state incapable of providing the circuit service. Also, since the user equipment is regarded as camping in the 3G network system though the packet switched domain, it recognizes itself as being incapable of providing the circuit service and continues to wait.
After all, the user equipment cannot perform the location area update and receive the circuit service until it receives normal system information block from the network system. Without receiving the normal system information block, the user equipment cannot use the circuit service but use only additional services, while call transmission/reception is impossible. This problem occurs not only when the system information block does not include circuit switched domain information, but also when the user equipment receives a system information block including inappropriate core network information.